High-purity monocrystals of such materials as silicon or germanium are grown in vacuum furnaces according to the Czochralski pulling technique by withdrawing a solidified crystal from a melt under precisely controlled conditions. The growing process is very slow, lasting several hours, during which the growing crystal is gradually lifted into a loading chamber. For example, the finished crystals, which can measure one meter or more in length, are typically grown at rates averaging between three and six centimeters per hour.
After completion of the growing process, the finished crystal remains within an evacuated environment of the loading chamber for several additional hours (e.g., six to nine hours) to cool. The furnace is ordinarily shut down during the cooling cycle (cooldown) to conserve power. After completion of the cooling cycle, the finished crystal is removed through a door in the loading chamber, and the furnace is cleaned and readied for another charge.
The loading chamber together with the top of the furnace is removed for cleaning the furnace interior (i.e., tank). This includes removing and replacing a crucible for containing the melt. Ten to fifteen percent of the original charge of crystal-forming material remains in the crucible; and its solidification during the cooling cycle causes the crucible, which is generally made of quartz, to break. Both the crucible and the scrap crystal-forming material are costly to replace.
After a new crucible is installed and filled with chunks of crystal-forming material (i.e., a new charge), the top of the furnace is replaced. In the meantime, a seed crystal is suspended within the loading chamber. The refitted furnace and loading chamber are evacuated (pumpdown), and the furnace is powered for melting the charge. The melting cycle (meltdown), which also lasts several hours, is continued until desired thermal conditions have been achieved in the melt for growing another crystal. The actual growing cycle is begun by dipping the seed crystal into the melt and slowly withdrawing the seed crystal from the melt. The melt crystallizes at the seed, and the new crystal grows to dimensions controlled by a rate of pulling the crystal from the melt.
Nearly one-half of the total processing time is spent on operations other than actual crystal growth. These operations include: cooling (cooldown), cleaning, reloading, evacuating (pumpdown), and melting (meltdown). The consequently large amount of processing time required for each crystal limits the productivity of crystal-forming machines, which are themselves expensive to purchase and maintain.
So-called "continuous" crystal-growing processes shorten the meltdown cycle by using a smaller initial charge and adding crystal-forming material during the crystal-growing process. However, special apparatus required to continuously supply charge to the melt adds more expense than can be saved from the shortened meltdown cycle. Nevertheless, "continuous" processes are used for achieving special crystal quality requirements, such as reduced oxygen content.
"Recharging" crystal-growing processes reuse the crystal and residual melt one or more times by continuously operating the furnace. After the cooled crystal is removed, a recharging device is installed in the loading chamber for gradually adding crystal-forming material to the melt until a full charge is reached. Then, the recharging device is replaced by a suspended seed crystal for starting the crystal growth.
While such "recharging" processes extend crucible life and reduce scrap of crystal-forming material, the savings is offset by the additional cost of operating the furnace throughout the cooling cycle of the crystal. Also, crucibles deteriorate with exposure to such high temperatures and add contamination to the melt. For example, heated quartz crucibles slowly dissolve in the presence of a silicon charge releasing silicon monoxide into the melt. The continuous dissolution of crucibles during such "recharging" processes diminishes crystal quality and limits the number of times the crucibles can be reused.
Crystal-growing machines are installed in clean rooms to limit contamination of high-purity crystals from air-borne particles, especially metal particles. The contaminants enter the crystal-forming material from brief exposure to ambient air while the machines are opened for cleaning and reloading. Market demands for larger diameter crystals (e.g., 200 mm-300 mm) increase problems with maintaining clean room environments. Larger machines require more space, which is more difficult to keep clean; and heavier duty lifting systems generate more contamination.